[blackbear_]

> I don't get why invest time in PhD if your work afterwards seems totally unrelated to your expertise

Depending on how the supervision chain is arranged, a PhD can a journey of discovery, of new science but first and foremost of yourself and your interests. It can be very self-directed and the only mandate is to discover something new. For this reason it is common for people to dip their toes in a few distinct but related subfields during those years until they find something that sticks (if at all), and the person that comes out of a PhD can be very different from the person who started it.